The invention relates to a structure having colossal magnetoresistive (CMR) properties, a method for producing such a structure and a device comprising a structure having colossal magnetoresistive properties.
Thin films having CMR properties are generally considered to have a high potential for application in e.g. reading heads of devices for magnetic storage of information. It has been revealed that this effect originates from the properties of grain boundaries. CMR has been observed in many perovskite ferromagnets.
Normally the employed magnetic fields are high, around 5 T. By use of such magnetic fields there can be a change of resistance of 80%. For a viable use of these materials in the information technology, CMR should occur on the application of small fields (0,1 T or less),
Research has been made on boundary-containing media, such as polycristalline and textured thin films, trilayer tunnel junctions, bridges on bicrystal substrates and ramp-edge junctions. It is possible to grow films on bicrystal substrates and to etch a bridge across the boundary, see for instance the document Applied Physics Letters, Volume 72, Number 16, xe2x80x9cMagnetoresistance of artificial La0,7Sr0,3MnO3 grain boundaries as a function of misorientation anglexe2x80x9d, S. P. Isaac, et al.
This document discloses a method of growing films on bicrystal substrates to form a single, well-controlled grain boundary at a specific misorientation angle at a known angle with respect to the chip. On either side of the bicrystal divide, the films grow epitaxially forming an artificial grain boundary across the centre of the chip. The films are patterned into a geometry with two sets of two identical arms. Two arms are centred on the substrate grain boundary. The highest values of the magnetoresistance have been obtained for a grain boundary with a misorientation angle of 45xc2x0. A drawback of bicrystal substrates is the limitation of the number of the bridges, which can be patterned on the boundary.
Furthermore, if single crystals are used instead of bicrystals, grain boundaries are produced in an uncontrolled manner, i.e. a polycrystalline film is obtained which is uncontrollably formed since the grains will have arbitrary sizes as well as orientations.
An object of the present invention is to overcome the drawback of prior art and to provide a magnetoresistive element and a method of producing a crystal structure having colossal magnetoresistance which is controllable.
A template is formed on a base crystal substrate material. The template exhibits different sections in which the crystallographic axis, or orientation, of a first section is different from the crystallographic axis, or orientation, of a second section. A colossal magnetoresistive film (CMR) is grown on the base crystal. Due to the different orientations, or misorientation, there will be a lattice misfit between the sections. A grain boundary is formed in the CMR film over the boundaries between the sections. The properties of the grain boundaries highly affect the magnetoresistive characteristics.
A large number of small islands can be patterned in the substrate, or in a seed layer deposited on the substrate, forming a template with an orientation different from that of the substrate. When a CMR film is grown epitaxially on such a surface, many grain boundaries will appear over the boundaries between the substrate template and the seed layer template. Their number can be controlled by varying the size and orientation of the templates. The grain boundary misorientation angle can be controlled by varying the material of the seed layer and the orientation of the template.
The in-plane orientation of a magnetoresistive film depends on the seed layer that is deposited on the substrate. There are seed layers on which CMR grows with the same in-plane orientation (cube-on-cube), e.g. SrTiO3, while on other seed layers the orientation of the deposited CMR film is rotated by some angle with respect to the substrate axis. As a result artificial grain boundaries are introduced in the CMR film. A large number of small islands can be patterned from a template in the seed layer to create sections with an in-plane orientation different from that of the substrate. The number of the grain boundaries inherited in the deposted CMR film depends on the size of the template element.
It is also possible to form magnetoresistive films that are formed by a template with sections having different out-of-plane orientation. When a CMR film is grown on a step in a substrate, one or several grain boundaries are formed over the step. If sections or islands are patterned in the substrate and a CMR film then is grown epitaxially, the grain boundaries that are created over the steps separate the CMR film into a two dimensional array of epitaxially grown areas with the same in-plane orientation. The number of grain boundaries depends on the substrate material, step angle and step height.
Preferably a single crystal substrate is used.